Various types of welding are known, such as manual welding, TIG (Tungsten Inert Gas) welding and MIG (Metal Inert Gas) welding. Various types of power supply apparatus are used for respective types of welding. One of the basic arrangements of such power supply apparatuses is as follows. A commercial AC voltage is rectified and smoothed into a DC voltage by an input-side rectifier and a smoothing capacitor. The DC voltage is then converted into a high-frequency voltage in an inverter and, then, transformed in a voltage-transformer. The transformed, high-frequency voltage is rectified in an output-side rectifier and is applied to a load. If occasion demands, the DC voltage from the output-side rectifier is converted into a low-frequency AC voltage before it is applied to the load. With this arrangement, a DC voltage is converted into a high-frequency voltage by an inverter, which permits the use of a small-sized voltage-transformer. As a result, the whole power supply apparatus can be made small.
A power supply apparatus for manual welding exhibits a fixed current output characteristic as shown by a curve A-2 in FIG. 2, in which output current is kept constant even when an output voltage changes. An output current setting device 2 for setting the constant output current is mounted on a control panel of a manual welding power supply apparatus as shown in FIG. 1. In manual welding, "hot starting" is employed, in which, as shown in a portion B-2 of FIG. 2, current larger than the fixed output current is provided to a welder for reliable initiation of arcing when the welding operation is to be started or when a load including a torch and a workpiece is short-circuited. A hot-start setting device 4 for setting current to flow during the hot start period is disposed on the control panel, too. A display 6, which displays an output voltage and output current, is also on the control panel.
DC TIG welding is suitable for welding, e.g. stainless steel. A power supply apparatus for a DC TIG welder has a fixed-current output characteristic as shown by a curve A-5 in FIG. 5. DC TIG welding may be "hot started", if occasion demands, as is understood from the characteristic portion B-5 in FIG. 5.
For welding a flat workpiece by TIG welding, the output current of the power supply apparatus is maintained constant as indicated by the curve A-5 in FIG. 5. However, if constant current is applied to the TIG welder when a workpiece, e.g. a pipe and, in particular, a round bottom portion of a horizontally disposed pipe is welded, melt may drop from the pipe, requiring another welding. Sometimes, such melt may adhere to the electrode of the TIG welder to make the electrode unusable.
To avoid such problems, as shown in FIG. 4, it has been proposed to apply a pulse current to the TIG welder in place of a constant output current. The pulse current shown in FIG. 4 comprises a base portion I.sub.B from which pulses I.sub.P extend repetitively. During each current portion I.sub.B, a molten pool formed on the bottom of the pipe is cooled to thereby prevent the melt from dropping or adhering to the welding electrode.
There are two methods for initiating an arc in TIG welding. One is to supply a small current to a welding electrode and a workpiece which are contacting each other, and, then, separate the electrode from the workpiece, which results in arcing between the electrode and the workpiece. This method is called "touch starting". In this specification, the welding with "touch starting" is called "touch-start welding". The other method is a "high frequency starting", in which a high-frequency, high-voltage is applied between a welding electrode and a workpiece spaced from each other. The voltage to be applied has a frequency of, for example, from 1 MHz to 3 MHz and a magnitude of, for example, from 5 KV to 20 KV. The application of such voltage causes an arc to be generated between the electrode and the workpiece. In this specification, the welding with "high frequency starting" is called "high-frequency-start welding".
Thus, as shown in FIG. 3, on a control panel of a DC TIG welder power supply apparatus, there are disposed, an output current setting device 8 for setting the magnitude of the output current, a hot start setting device 10, and an upslope and downslope time setting device 12. The upslope and downslope time setting device 12 is used to set an upslope time TU required for a pulse output current to rise from a initiating current Id to a peak pulse current, i.e. the set output current I.sub.P and to set a downslope time TD required for the pulse output current to decrease from the peak current I.sub.P down to a crater current I.sub.C which flows at the end of the welding operation. Also, a pulse frequency setting device 14 for setting the frequency F (=1/T) of the pulse current, a pulse switch 16 for switching the supplied current between a pulse current and a DC current, an arc starting mode switch 18 for switching the starting mode between the touch starting and the high frequency starting, and a display 20 for displaying the output voltage and the output current are disposed on the control panel.
AC TIG welding is another TIG welding. There is a power supply apparatus for use with a TIG welder operable from both AC and DC (hereinafter referred to AC/DC TIG welder). DC TIG welding using an AC/DC TIG welder power supply apparatus is the same as the DC TIG welding with the above-described DC TIG welding power supply apparatus.
AC TIG welding may be used for aluminum welding. A workpiece of aluminum typically has an oxide film of a high melting point thereon. Therefore, if a DC power supply apparatus is used to supply current, with the workpiece being positive and with the welding electrode being positive, the temperature of the workpiece cannot rise enough. Accordingly, the workpiece cannot be welded. On the other hand, if current is supplied, while the electrode is held positive with the workpiece being negative, thermoelectrons are discharged from the workpiece, and the thermoelectrons remove the oxide film from the surface of the workpiece, which is called an cleaning effect, so that it becomes possible to weld the workpiece. However, if the workpiece kept positive when current is supplied, the electrode can be cooled. Then, the AC TIG welding, in which an AC voltage is applied between the workpiece and the torch or welding electrode, has both cleaning and electrode cooling effects. The cleaning effect and the cooling effect can be optimized by appropriately adjusting the ratio of the time t1 during which current is supplied with the workpiece being positive and with the electrode being negative, to the time t2 during which the workpiece is placed negative (see FIG. 7).
As shown in FIG. 6, on a control panel of an AC/DC TIG welder power supply apparatus, there are disposed an output current setting device 22, a hot-start setting device 24, an upsloping and downsloping time setting device 26, a pulse frequency setting device 28, a pulse switch 30 for providing or removing pulse current, a starting mode switch 32 for switching the arc starting mode between the touch starting mode and the high-frequency starting mode, all for DC TIG welding. Also disposed is a display 34 for displaying the magnitudes of output voltage and current. In addition, a frequency setting device 36 for setting the frequency of the voltage and a duty ratio setting device 38 for setting the ratio of the positive to negative portions of the pulse voltage, both for the AC TIG welding are also disposed on the control panel.
MIG welding is used for welding steel plates. A welding wire is forwarded to a workpiece by a wire feeding machine, and a voltage is applied between the workpiece and the wire. The wire is brought into contact with the workpiece, so that current flows between them to generate Joule heat. The Joule heat melts the wire portion which is in contact with the workpiece. The molten wire material is separated from the wire and drops onto the workpiece. As the wire is separated from the workpiece, an arc is generated between them. The wire is heated and melted by the arc to produce a molten droplet of the wire material, which short-circuits the wire and the workpiece. Then, the droplet drops onto the workpiece, so that the wire and the workpiece are separated from each other, causing an arc to be generated between them. During this process, the wire feeding machine continues to feed the wire. This process is repeated, so the wire melts to weld the workpiece. A power supply apparatus for use in MIG welding has a constant-voltage supplying characteristic.
In the MIG welding, it is necessary to suppress a large current which would flow when the wire and the workpiece are short-circuited by a molten wire material. On the other hand, it requires a reactor having a small reactance which allows a large current to flow when an arc is initially generated. Accordingly, in MIG welding, a small reactance reactor is used, while suppressing the current flowing during short-circuiting.
For this purpose, a power supply apparatus for MIG welding includes, as shown in FIG. 8, an output current setting device 40, a short-circuiting current setting device 42 for setting the magnitude of the current flowing when the workpiece and the welding wire are short-circuited, a welding wire feeding rate setting device 44 and an output voltage and current display 46. These devices are all disposed on a control panel of the power supply apparatus.
A single power supply apparatus which would be used for all of the above-described various types of welding would be complicated in structure, because of the necessity of many setting devices and of the complicated setting operation of the respective setting devices.
Therefore, an object of the present invention is to provide a power supply apparatus which can be easily adapted for use with various types of welding. Another object of the present invention is to provide such a power supply apparatus in which various settings required for any specific welding can be made relatively easily.